Diversified farms bolster forest-bird populations despite ongoing declines in tropical forests

Significance

While diversified farms provide habitat for many species, it is unclear whether agricultural areas can support viable populations of species that are sensitive to environmental changes. We explored this question by tracking changes in bird populations in different farming systems over 18 y in Costa Rica. We found that diversified farming practices can support the long-term population growth of many sensitive forest-affiliated and insect-eating species over time. Unexpectedly, population declines in surrounding forest habitats outweigh the increases in diversified farms. Our findings suggest that the benefits of diversified farming practices for biodiversity can accrue through time and that such practices retain vital potential for future restoration.

Abstract

While some agricultural landscapes can support wildlife in the short term, it is uncertain how well they can truly sustain wildlife populations. To compare population trends in different production systems, we sampled birds along 48 transects in mature forests, diversified farms, and intensive farms across Costa Rica from 2000 to 2017. To assess how land use influenced population trends in the 349 resident and 80 migratory species with sufficient data, we developed population models. We found, first, that 23% of species were stable in all three land use types, with the rest almost evenly split between increasing and decreasing populations. Second, in forest habitats, a slightly higher fraction was declining: 62% of the 164 species undergoing long-term population changes; nearly half of these declines occurred in forest-affiliated invertivores. Third, in diversified farms, 49% of the 230 species with population changes were declining, with 60% of these declines occurring in agriculture-affiliated species. In contrast, 51% of the species with population changes on diversified farms showed increases, primarily in forest-affiliated invertivores and frugivores. In intensive farms, 153 species showed population changes, also with similar proportions of species increasing (50%) and decreasing (50%). Declines were concentrated in agriculture-affiliated invertivores and forest-affiliated frugivores; increases occurred in many large, omnivorous species. Our findings paint a complex picture but clearly indicate that diversified farming helps sustain populations of diverse, forest-affiliated species. Despite not fully offsetting losses in forest habitats, diversified farming practices help sustain wildlife in a critical time, before possible transformation to nature-positive policies and practices.